Volume resonators/oscillators that operate with acoustic waves, known as thin-film bulk acoustic resonators (FBAR), or also called bulk acoustic wave (BAW) resonators, are based on a piezoelectric basic element that is provided with an electrode on each of two main surfaces. Such a resonator has a resonant frequency fr that is dependent on the thickness L0 of the basic element, measured between the electrodes, in accordance with the formulafr=v/2L0.
Here, v designates the velocity of the longitudinal wave in the piezoelectric basic element. Such resonators can, for example, be used in the construction of RF filters. The layer thickness required for the basic element (in the μm range) in this frequency range means that thin-film methods are required for the manufacture of the basic element. In order to use the FBAR resonators for RF filters, and to achieve a high yield, a high degree of homogeneity of the layer thickness, with a maximum deviation of less than 0.3%, is required, in order both to obtain a sharp resonant frequency and to obtain the position of the resonant frequency at the desired location. However, using currently available layer deposition technologies, this layer thickness homogeneity cannot be achieved on substrates having diameters greater than 150 mm.
There do exist methods for treating a layer manufactured in thin-film technology through subsequent trimming (i.e., subsequent correction of the layer thickness, and thus of the resonant frequency) until a desired layer thickness having a desired degree of homogeneity is obtained. These methods include additional deposition of material or subsequent removal of material. However, these methods are not yet mature enough for profitable mass production, and are also expensive.
Today, primarily two paths are known for tuning an FBAR resonator to a desired resonant frequency. If it is economically feasible for small-scale individual applications, the resonators can, as explained above, be trimmed to a desired resonant frequency individually, through subsequent layer deposition or subsequent ion-beam etching. Alternatively, through an external network matching, the resonators can be provided with external electrical switching elements, in particular with L and C elements, in order to set the desired resonant frequency fr. However, this results in a high piece price for the filter, and moreover can be carried out only if the filter volume does not represent a critical size for use in telecommunication, as a sensor element, and in control circuits.